A need exist for an improved electromagnetic transmitting device which can be used, for example, for tracking and locating pigging devices employed in pipeline systems. The improved electromagnetic transmitting device will preferably be (a) compact enough to be installed within or towed behind pigging devices used even in small diameter lines, (b) pressure sealed and pressure resistant, (c) impact resistant, (d) corrosion resistant for use in water lines, petroleum and petroleum product lines, gas lines, etc., (e) resistant to cleaning agents and other chemicals used in such lines, and (f) economical to build and use.
An example of a prior art electromagnetic transmitter is provided in U.S. Pat. No. 6,088,580. The prior art transmitter disclosed in U.S. Pat. No. 6,088,580 comprises a ferromagnetic tubular battery housing. The ferromagnetic battery housing has an electromagnetic coil wrapped around the exterior surface thereof so that the battery housing serves as the transmitter core. The electronic transmission components for the prior art device are also positioned on the exterior surface of the battery housing, adjacent to the electromagnetic coil. In order to provide some degree of protection to the electromagnetic coil and the electronics on the exterior of the battery tube, these components are covered with a non-conductive plastic coating.
Unfortunately, the prior art transmitter disclosed in U.S. Pat. No. 6,088,580 is deficient in various significant respects. In one respect, the only protection provided for the electromagnetic coil and the electronic components of the prior art transmitter against damage caused, for example, by abrupt stops, collisions with valve gates, other impacts, and the high fluid pressure conditions often encountered in pigging operations, is merely a non-conductive plastic coating. The electromagnetic coil and the electronic components are therefore susceptible not only to destructive damage caused by physical collisions and impacts, but also to corrosion and interference caused by exposure to pipeline fluids which can reach the coil and electronic components (a) through breaks, cracks, or chips which occur in the plastic coating and/or (b) by infusion beneath the plastic coating due to high pressure conditions existing in the pipeline.
Moreover, the ferromagnetic core tube of the prior art transmitter is also susceptible to corrosion and chemical attack. Because the core tube must have significant ferromagnetic properties, it cannot be formed, for example, of an austenitic stainless steel such as 304 stainless or 316 stainless which is essentially non-magnetic, or even a ferritic stainless steel which is only somewhat magnetic and is also less rugged.
Rather, it is recommended in U.S. Pat. No. 6,088,580 that the ferromagnetic core tube of the prior art transmitter be formed simply of low carbon steel. Although possessing ferromagnetic properties, low carbon steel is highly susceptible to corrosion when exposed to water and other fluids. Consequently, the core tube of the prior art transmitter is also subject to damage and corrosion caused by exposure to water and/or other fluids or chemicals present in the pipeline which breach the plastic coating through breaks, cracks, or chips, or by pressure infusion. In addition, even if the plastic coating remains intact, the capped end of the ferromagnetic core tube, which must be openable for installing and replacing the batteries, is not coated with the plastic material and therefore is constantly exposed to whatever fluids and chemicals happen to be in the line, no matter how corrosive.